1. Field of the Invention
The present invention relates to an optical scanning apparatus suitable for use in a laser beam printer, a digital copy machine, a multifunction printer, etc., that perform electrophotography processes, and an image-forming apparatus including the optical scanning apparatus.
2. Description of the Related Art
In a conventional optical scanning apparatus for a laser beam printer, a copy machine, etc., a light beam that is optically modulated in accordance with an image signal is emitted from a light source, is periodically deflected and scanned by a rotating polygon mirror, and is guided toward the surface of a recording medium (photosensitive drum).
The light beam deflected and scanned by a deflecting surface of the polygon mirror is caused to form a spot on the surface of the photosensitive recording medium (photosensitive drum) by an fθ lens. Thus, image recording is performed by optically scanning the surface of the recording medium.
FIG. 10 is a schematic diagram illustrating the main portion of a conventional optical scanning apparatus.
Referring to FIG. 10, a divergent light beam is emitted form a light source 1 and is collimated by a collimating lens 2. Then, the collimated light beam is incident on a cylindrical lens 4 having a predetermined refractive power only in a sub-scanning direction after the width of the light beam is restricted by a diaphragm 3.
The collimated light beam incident on the cylindrical lens 4 exits the cylindrical lens 4 without a change in a main-scanning cross section (plane).
The light beam converges in a sub-scanning cross section (plane), thereby forming a line image on a deflecting surface (reflective surface) 5a of the polygon mirror 5.
The light beam is deflected and scanned by the deflecting surface 5a of the polygon mirror 5 and is guided toward a photosensitive drum surface 8, which functions as a surface to be scanned, through an fθ lens 6. The polygon mirror 5 is rotated in the direction shown by the arrow A, so that the photosensitive drum surface 8 is optically scanned in the direction shown in the figure and image information is recorded thereon (refer to, for example, Japanese Patent Laid-Open No. 04-321370 and Japanese Patent Laid-Open No. 2002-40350).
In general, a semiconductor laser capable of direct modulation is used as a light source for an optical scanning apparatus.
Power of light emitted from the semiconductor laser varies depending on heat emitted from the semiconductor laser itself and environmental variation (for example, ambient temperature variation).
Therefore, generally, the power of light emitted is constantly detected and automatic power control is performed such that the power of the light beam emitted from a light-emitting portion of the light source is always maintained constant.
Various automatic power control methods for the semiconductor laser are suggested and put into practical use.
For example, in a first method, which is most commonly used, a rear light beam that is emitted from a semiconductor laser in a direction opposite to the direction in which an image-drawing light beam is emitted (i.e., a light beam emitted from a rear side of a semiconductor substrate) is detected and used for the light-power control.
The image-drawing light beam is a light beam used for forming dots in an image effective area on the photosensitive drum surface.
According to the first method, a photosensor, which functions as a light-power detector, can be installed in a package of the laser light source. Therefore, the overall size is relatively small and the light-power control for the light source can be easily performed.
However, since a light beam other than the image-drawing light beam is monitored by the photosensor and the influence of heat emitted by the light source is significant, it is difficult to perform high-accuracy light-power control (automatic power control).
In addition, it is difficult to apply the first method to a light source like a vertical cavity surface emitting laser (VCSEL) that does not emit a rear light beam.
On the other hand, recently, the vertical cavity surface emitting laser has been attracting attention as a light source for an optical scanning apparatus. Compared to a conventional edge emitting laser, the vertical cavity surface emitting laser is characterized in that the number of light-emitting points can be considerably increased, two-dimensional parallel integration is possible, and the layout of the light-emitting points is easy.
In the vertical cavity surface emitting laser, light is emitted in a direction perpendicular to the semiconductor substrate. Therefore, the rear light beam is basically not emitted and it is difficult to use the light-power control method in which the rear light beam is used.
Japanese Patent Laid-Open No. 04-321370 discusses a second method in which the light-power control is performed using a portion of a light beam emitted from a semiconductor laser that is blocked by an aperture diaphragm.
According to the method discussed in Japanese Patent Laid-Open No. 04-321370, light-power control (automatic power control) can be performed without being affected by heat emitted from the light source.
However, as the utilization ratio of the image-drawing light beam is increased, the power of light that can be used for the automatic power control is reduced in inverse proportion to the utilization ratio.
In addition, although the light used for the automatic power control is not the rear light beam used in the first method, the blocked light beam, which is different from the image-drawing light beam in practice, is used and it is still difficult to perform high-accuracy light-power control.
Japanese Patent Laid-Open No. 2002-40350 discusses a third method in which light-power control (automatic power control) is performed by separating a portion of a light beam that travels from a light source to a deflecting unit with a half mirror and guiding the separated portion toward a light-receiving element (photosensor).
According to the method discussed in Japanese Patent Laid-Open No. 2002-40350, a portion of the actual image-drawing light beam is used. Therefore, high-accuracy automatic power control can be performed.
However, since a portion of the image-drawing light beam is separated, a loss occurs in the power of the image-drawing light beam.
In addition, an expensive optical element, such as a half mirror, is required for separating a portion of the image-drawing light beam.
In particular, when the above-described vertical cavity surface emitting laser is used, high-power light emission is basically difficult compared to the case in which the edge emitting laser is used. Therefore, the loss in the power of the image-drawing light beam caused when a portion of the image-drawing light beam is separated and detected by the light-receiving element is a serious problem.
The output of the edge emitting laser is generally several tens of milliwatts, whereas the output of the vertical cavity surface emitting laser is generally less than several milliwatts. Therefore, when the vertical cavity surface emitting laser is used, the loss in the power of the image-drawing light beam is a serious problem.